When I install 6 it created a New Catalog Converting the LR 5 Catalog to a LR 6 Catalog. So It would seem Adobe has tweaked the catalog and it is not backward compatible.
Yes. I found the same thing. When I installed LR 6, it imported my current working LR 5 Catalog, naming it "catalog-x-x-2.lrcat" (x-x are the LR 5 version); i.e. it appended the "-2" to the LR 5 version. When I tried to open the "-2" version in LR 5.7 I received an error message saying the "-2" version was not compatible.
Can I continue to sync the resp directories between LR 5.7 and LR 6 back and forth?
Thanks for your thoughts!
No. You can upgrade an LR 5 catalog to LR 6 but you cannot go back the other way. There really is no work around for this. You're going to have to stay on the same version on all your computers.
The short answer is that the LR5 and LR6 catalogs are not compatible.